Class BooleanControl

A BooleanControl provides the ability to switch between
two possible settings that affect a line's audio. The settings are boolean
values (true and false). A graphical user interface
might represent the control by a two-state button, an on/off switch, two
mutually exclusive buttons, or a checkbox (among other possibilities).
For example, depressing a button might activate a
MUTE control to silence
the line's audio.

As with other Control subclasses, a method is
provided that returns string labels for the values, suitable for
display in the user interface.